Yes, you can actually change out the image in \Track-It!\SelfService\Content.[version number]\images\Common. The file is called "LoginBackground.jpg". If you replace that (and clear your cache) then your self-service login will be replaced.

That's something you can do with the technician login as well. That lives in \Track-It!\Application Server\Content.[version number]\images\Common. And there it's the file called "TI-login-cover.JPG".

Only note to make is that it doesn't survive an upgrade, so after every upgrade you need to replace the files again.